In recent years, there have been proposed and developed various hydraulically-operated vane member equipped variable valve timing control devices, capable of locking a vane member at an intermediate position between a maximum phase-advance position and a maximum phase-retard position by means of a lock mechanism during a starting period of an internal combustion engine. To release a locked state of the vane member with a lock pin of the lock mechanism engaged, working fluid (hydraulic oil) in either a phase-advance chamber or a phase-retard chamber is used. During rotation of a camshaft, owing to reaction forces from the engine valve side to cams, a so-called alternating torque (in other words, positive and negative torque fluctuations) acts on the camshaft. Owing to alternating torque transmitted from the camshaft, the vane member tends to flutter, and thus hydraulic-pressure fluctuations in the phase-retard chamber and the phase-advance chamber occur. Owing to such hydraulic-pressure fluctuations, arising from alternating torque, there is a possibility that the locked state cannot be easily released.
To avoid this, Japanese Patent Provisional Publication No. 2003-247403 (hereinafter is referred to as “JP2003-247403”) teaches that an exclusive oil passage, only used for the lock mechanism, is provided separately from supply-and-exhaust oil passages for phase-advance chambers and phase-retard chambers, and a single control valve is also provided for working-fluid supply-and-exhaust control for phase-advance chambers and phase-retard chambers and for hydraulic-pressure control for the lock mechanism for locking or unlocking.